1. Field of the Invention
The present invention relates generally to vacuum fluorescent display systems of the type which include one or more cathode filaments, a plurality of segmented anodes situated a prescribed distance from the cathode filament, and a plurality of control grids interposed between the cathode filament and each of the segmented anodes for sequentially illuminating selected segments of the anodes to thereby display desired characters. More particularly, the present invention relates to means and method for driving the above described display system which in response to the driven and undriven states of the control grids, controls the application and removal of heating power to the cathode filament whereby the voltage drop along the filament is eliminated when the control grids are driven thereby substantially eliminating variations in luminous intensity from one anode to another.
Generally speaking, various embodiments of the improvement of the present invention either remove heating power from the cathode filament in response to a driven state of the control grids or apply heating power to the cathode filament in response to an undriven state of the control grids.
2. Description of the Prior Art
The conventional multidigit or multicharacter vacuum fluorescent display is fundamentally a plurality of triode vacuum tubes wherein each vacuum tube shares a common cathode filament and each further includes an anode (segmented) and a control grid. In a multidigit numerical display system each anode is divided into a plurality of segments which are arranged in a pattern that will allow all numerical digits (0 through 9) to be displayed by using combinations of these segments. The surfaces of the anode segments are typically coated with a fluorescent material which emits a blue green light when impacted with electrons.
When an appropriate electrical voltage is applied across the cathode filament, the filament is heated to a temperature at which electrons are thermally emitted. If a positive voltage is applied to the anode and control grid, the thermal electrons emitted from the filament are accelerated by the electric field formed by the anode segments and control grid. These electrons impact the anode after passing through the grid thereby exciting the fluorescent material causing it to emit light. When the anode or control grid voltage is negative, the electrons are repelled and no light is emitted.
If a positive voltage is applied to a combination of anode segments corresponding to a digit or character to be displayed and a positive voltage is simultaneously applied to the control grid corresponding to that anode, a desired digit or character will be displayed from the combination of lighted anode segments.
In the conventional multidigit display system each digit is sequentially illuminated by repeatedly applying a positive voltage to the appropriate control grids and selected anode segments while maintaining all other grids and anode segments at a negative voltage. The persistence of the human eye makes all of the digits appear to be continuously illuminated provided that the repetition rate of illumination of each digit is high enough.
Typically, a separate power source is required in order to heat the cathode filament. However, unlike conventional vacuum tubes, vacuum fluorescent display cathode-anode voltages are very low. Accordingly, the cathode filament voltage is not insignificantly small relative to the cathode-anode voltage as in a conventional vacuum tube. Different portions of the filament are at different potentials due to the drop in voltage experienced along the filament. Since different anode segments representing different digits utilize different portions of the filament, the cathode-anode voltage drop and the cathode-grid voltage drop vary from anode to anode or digit to digit. These voltage variations can cause intensity variations from digit to digit.
A conventional way to eliminate this variation in luminous intensity is to apply an AC voltage across the cathode filament in such a way as to time-average the variations in luminous intensity at a rate too fast for human perception. In a multidigit display system the frequency of the system drive signal and the frequency of the cathode filament signal (AC power line frequency or DC-DC converter frequency) are typically asynchronous and any beat frequencies between the two frequencies are arranged so that they are unperceptable. Many times it is desirable that the frequencies of the system drive signal and the AC power line frequency be synchronous. Typically when this condition exists, beat frequencies with perceivable amplitudes result in flicker and static intensity variations from digit to digit may also appear. Accordingly, a need exists for a drive means wherein the AC line power freuency and the system drive frequency or frequency of illumination of the digits of a vacuum fluorescent display system operate synchronously and variations in luminous intensity from digit to digit are substantially eliminated.